Commonly held U.S. Pat. No. 5,543,532, hereby fully incorporated by reference, discloses cation substituted VPO catalysts, preferably in the form of solid solutions, which are useful as catalysts for the oxidation of alkane hydrocarbons.
A review of the improvements made in this technology is given by G. J. Hutchings, Applied Catalysis, 72 (1991), Elsevier Science Publishers B. V. Amsterdam, pages 1-31. The preferred method of preparation of VPO catalysts is the hydrochloric acid digestion of V2O5 and H3PO4 in either an aqueous solvent, as described, for example, in U.S. Pat. No. 3,985,775, or non-aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, vanadium hydrogen phosphate, VO(HOPO4).(H2O)0.5.
The precursor is then activated by heating, as described, for example, in U.S. Pat. No. 3,864,280 and U.S. Pat. No. 4,043,943. Further optimization of the preparation is described in U.S. Pat. No. 4,132,670, whereby vanadium pentoxide is heated with a selected anhydrous unsubstituted alcohol, adding an orthophosphoric acid to form the catalyst precursor and calcining the precursor to obtain the catalyst having high intrinsic surface area. Further attempts to improve the VPO catalyst performance by the use of dopants and/or supports are described in U.S. Pat. No. 4,442,226 and U.S. Pat. No. 4,778,890.
Vanadium, phosphorus reagents and oxygen can form a large number of distinct compounds which have been well characterized, e.g., alpha-VOPO4, gamma-VOPO4, VOHPO4, (VO)2P2O7, VO(PO3)2 and VO(H2PO4)2. The most active catalytic phase is believed to be (VO)2P2O7, which is also the predominant oxide phase in VPO catalysts. Nevertheless, VPO catalysts are usually referred to as “mixed oxides” in recognition of the probable presence of other oxide phases. VPO catalysts typically have V:P atomic ratios in the range of 1:1 to 1:2 and have an average bulk vanadium oxidation state in the range of 4.0-4.3.
G. J. Hutchings and R. Higgins disclose in J. Catalysis 162, 153-168 (1996) methods whereby hydrate salts were physically deposited by incipient wetness onto vanadium hydrogen phosphate precursor. However, there is no distinction made between aqueous and non-aqueous impregnations, nor is reactive grafting discussed. The present invention has found very large differences in examples where nitrate salts and ammonium molybdate (in water) are used versus examples in which alkoxides (in non-aqueous solvents) are used. The differences are apparent both in the differences in reactivity toward hydrocarbon reactants (e.g., n-butane) and in the microstructure of the catalyst.